Moreover, using Cookie Monster you can allow cookies for second level domain names. Meaning that if you navigate to mozilla.org, cookies for www.mozilla.org will be accepted — but cookies for adnetwork.com will not.

This feature makes many sites work which otherwise break with third party cookies disabled, while still discarding the majority of third party cookies.

Firefox has this feature now build in. Goto privacy -> custom settings -> cookies from thierdparty and change to sites i have visited.

I don't think that that option accomplishes the same, or is even similar.

What about sites that you never visit (= type in address bar / follow links to) directly, but which are on some subdomain of the visited site nonetheless? Cookiemonster will accept the cookie. Firefox, with your config, will not.

What about sites that you do visit (eg facebook.com) but don't want to accept/send third party cookies for? Firefox, with your config, will. Cookiemonster won't.

I also use this add-on. While FF has the options too, the add-on allows me to easily access all the features and see all the details that I want to see.

The most important feature is that it's easy and quick.

...which is a built-in Firefox feature.

... a built-in Firefox feature with an epically bad UI. If you set Firefox to ask whether to accept cookies from a domain and whether to keep them beyond the end of the session, you get asked per cookie not per domain the first time you visit a site. And FF presents the questions that pop up in a stack of modal dialog boxes, potentially dozens, and sometimes they appear out of order so you have to dig around to find the one that's willing to accept a click, which can be difficult on account of them all being modal.

Firefox does allow you to keep a white-list of cookie-enabled sites, but you have to go to the Options menu to do this. Cookie Monster let's you set cookie permissions for the page you're currently visiting by clicking on an icon in the status bar. The icon also gives you a visual indication of what the current page's settings are. It's essentially a nicer user interface on top of the built-in Firefox functionality.